1. Field of the Invention
The present invention relates to an isolated switching power supply apparatus in which there is substantially no period during which energy transmission between the primary side and secondary side of a transformer is not performed.
2. Description of the Related Art
To date, known examples of general isolated switching power supply apparatuses include forward converters and flyback converters. All of these isolated switching power supply apparatuses store energy in a transformer or an inductor while the main switching device is on, and thereby transmit energy from the primary side to the secondary side while the primary side main switch is either on or off. Hence, there exists a period during which energy is not transmitted from the primary side to the secondary side while the main switch is either off or on.
A known example of an isolated switching power supply apparatus in which energy is transmitted from the primary side to the secondary side during both the on period and off period of the primary side main switching device is a two-transformer DC-DC converter having two transformers. An example thereof is disclosed in Japanese Unexamined Patent Application Publication No. 2005-51994 described below.
Referring to FIG. 1, as the primary side circuit of Japanese Unexamined Patent Application Publication No. 2005-51994, a series circuit constituted by a primary coil W1 of a transformer T1, a primary coil W4 of a transformer T2, and a main switch Q1 is connected to an input DC power source 2 at connection nodes 10 and 20.
A series circuit constituted by a primary coil W5 of the transformer T2, a primary coil W2 of the transformer T1, and a capacitor C1 is connected between the connection node of the primary coil W4 of the transformer T2 and the main switch Q1 and the connection node 20 of the minus terminal of the input DC power source 2 and the main switch Q1.
Further, a series circuit constituted by the capacitor C2 and a sub switching device Q2 is connected between the connection node of the primary coil W4 of the transformer T2 and the main switching device Q1 and the connection node of the primary coil W2 of the transformer T1 and a capacitor C1.
As the secondary side circuit, a series circuit constituted by a secondary coil W3 of the transformer T1 and an output switch Q4 and a series circuit constituted by an output switch Q3 and a secondary coil W6 of the transformer T2 are connected in parallel between the two ends of a load system 3. The output switches Q3 and Q4 function as synchronous rectifier devices, whereby a center-tap full-wave rectifier circuit is formed. A capacitor C3 is connected between the two ends of the load system 3 as a smoothing capacitor.
In this manner, the sub switching device Q2 is off while the main switching device Q1 is on, and on the primary side, a current flows through the primary coil W1 of the transformer T1 and the primary coil W4 of the transformer T2. On the secondary side, the output switch Q3 is on and the output switch Q4 is off; a current flows through the secondary coil W6 of the transformer T2; and an output voltage is applied to the load system 3.
The sub switching device Q2 is on while the main switching device Q1 is off, and on the primary side, a current flows through the primary coil W2 of the transformer T1 and the primary coil W5 of the transformer T2. On the secondary side, the output switch Q3 is off and the output switch Q4 is on; a current flows through the secondary coil W3 of the transformer T1; and an output voltage is applied to the load system 3.
However, the isolated switching power supply apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2005-51994 is characterized in that a choke coil is not required as a result of two transformers having been provided, and hence, has a problem in that two transformers are needed, resulting in an increased size of the circuit.
In addition, although the advantage of no choke coil is necessary for allowing energy to be transmitted from the primary side to the secondary side both when the main switch Q1 is on and off is effective, there is a problem in that no choke coil on the secondary side causes an output ripple to increase due to switching noise generated during the on/off switching period of the main switching device Q1 on the primary side, resulting in an increased size of the smoothing capacitor C3.
Further, since a voltage which is the sum of an input voltage Vin and a voltage across the capacitor C2 is applied between the drain and source of the main switching device Q1, a high-voltage switching device is required. A high-voltage switching device has a large on resistance, which is a resistance component during conduction, and hence, conduction loss in the switching device increases. This results in a decrease in efficiency and an increase in cost.